Presently, U.S. energy consumption statistics are overwhelming: about 20 million barrels of oil a day are used, of which 70% go toward transportation. With oil prices averaging $100 per barrel, the U.S. spends about 2 trillion dollars a year on petroleum-based energy. More than half of the oil used is imported, so the United States is still “energy dependent.” Less than 10% of the U.S.'s energy comes from renewable resources; of that, 2% is from biofuels.
U.S. federal policy on biofuel usage is outlined in the Renewable Fuel Standard (RFS) which provides the minimum biofuel volume to be blended into the national fuel supply each year. Initially, RFS 1—also known as the EPA Act of 2005—mandated a minimum use of 4 billion gallons of biofuel in 2006 and, by 2012, a minimum of 7.5 billion gallons. RFS 2—also known as the EPA Act of 2007—mandated minimum biofuel blending volume of 9 billion gallons by 2008 and 36 billion gallons by 2022. Of this, corn biofuel is limited to a maximum of 15 billion gallons; however, a minimum of 16 billion gallons of cellulosic biofuel is mandated.
The United States is the world's largest producer of fuel ethanol and corn is the primary feedstock for ethanol production. In January 2012, 14.9 billion gallons of ethanol were produced in the U.S. and 42.5 million short tons of coproduct—namely dried distillers grain with solubles—were produced. As the U.S. is close to producing the maximum volume of corn biofuel mandated by RFS 2, the efficient production and commercialization of cellulosic ethanol are subjects of intense research. Although some agronomic hurdles to the production of lignocellulosic feedstock exist, the most significant limitations to commercial production of cellulosic ethanol are associated with the costs of enzymes for saccharification, logistics, and biomass processing. For example, due to low bulk density and high volumes, transportation of biomass over long distances is uneconomical. Enzymes for large-scale biomass saccharification are estimated to cost $2-$3 per gallon of biofuel. Enzymes are used at high loading levels as the conversion efficiency of these enzymes is much lower than the starch hydrolyzing enzymes used in producing ethanol from corn.
β-glucosidases and β-xylosidases are extremely important terminal enzymes involved in producing fermentable sugars such as glucose and xylose from non-fermentable intermediates such as cellobiose, cello-oligosaccharides, xylobiose, and xylo-oligosaccharides. Strains of Trichoderma fungus are the most common cellulase-producing microorganisms currently employed in an industrial setting. However, commercial strains typically lack the high levels of β-glucosidases required for efficient hydrolysis of cellobiose. Commercial cellulose preparations are typically supplemented with β-glucosidases to increase the rate of cellulose hydrolysis. High levels of β-glucosidases increase conversion of lignocellulosic material into glucose and decrease the inhibitory effect of cellobiose on endo- and exo-cellulases, in turn leading to greater ethanol yield. β-glucosidases also serve several other important biological functions including, but not limited to, cellulase induction, deglycosylation of isoflavone glycosides, and synthesis of industrially-important glycosidic compounds. Currently, the Trichoderma strain most commonly used in industry is the extensively genetically-manipulated Trichoderma reesei RUT-C30. However, this strain, while hypercellulolytic in nature, exhibits relatively low levels of β-glucosidase and β-xylosidase activity.
Agricultural residues like corn stover and wheat straw, bioenergy crops, and residues from the forestry and logging industries, all constitute biomass feedstock for ethanol, and many of these are abundant on local farms. The U.S. has the capacity to produce about 1.3 billion dry tons of sustainably collectable biomass annually, allowing the country to meet a third of its energy demand by 2030 while also continuing to meet food, feed, and export demands according to the US-DOE Billion ton study (2005) and Billion ton update (2011). These estimates take into consideration the sustainable production of lignocellulosic feedstock at baseline and under high yield scenarios for energy crops by state and county, secondary resources by state, and primary herbaceous non-woody and woody crops by county. For example, if biomass availability at high yield was assumed at $60 per dry ton, then 1 billion dry tons of biomass would be easily available by 2022 at a 4% growth rate, which would provide close to a trillion kilowatts of power by 2022. The U.S. has about 922 million acres of farmland. In 2006, average farm size by state varied from as high as 2,610 acres in Arizona to 71 acres in Rhode Island.
Presently, the U.S. generates about 400 to 600 million dry tons of biomass annually for potential fuel production. Transport of this biomass to fuel processing facilities is costly and current methods for processing biomass into fuel are cost-prohibitive and beyond the technical capabilities and resources of farm workers, owners, and managers.